Specifically, researchers have figured out why some people get fat when they eat too much and other people don't get fat, even when they eat the same amount:

The people who get fat get fat because they sit around all day. The people who don't get fat don't sit around as much.

Importantly, the difference between the fatties and the non-fatties in the study had nothing to do with exercise. None of the folks in the "inactivity" study were allowed to exercise. The folks who didn't get fat didn't exercise--they just didn't spend as much time sitting. Instead, they stood. They walked. They took stairs instead of elevators. They fidgeted. Etc.

Electrical activity in the muscles drops — “the muscles go as silent as those of a dead horse,” [inactivity researcher Marc] Hamilton says — leading to a cascade of harmful metabolic effects. Your calorie-burning rate immediately plunges to about one per minute, a third of what it would be if you got up and walked. Insulin effectiveness drops within a single day, and the risk of developing Type 2 diabetes rises. So does the risk of being obese. The enzymes responsible for breaking down lipids and triglycerides — for “vacuuming up fat out of the bloodstream,” as Hamilton puts it — plunge, which in turn causes the levels of good (HDL) cholesterol to fall.

Another bummer: You can't counteract the harmful effects of sitting by exercising once in a while. If you exercise a lot but also sit around a lot, you'll still have a shorter lifespan than people who don't sit so much.

But here's the good news:

You don't have to start running marathons to offset all that sitting. You also don't have to get a treadmill at the office and sweat all day. You just have to get up every hour or two and walk around for a while. Or, alternatively, you just have to stand instead of sit.